As electric and hybrid electric vehicles (EV or HEV, which will be collectively called electric drive vehicles or EDV) vehicle retire, they introduce large quantities of retired batteries. Old EDV batteries that are no longer suitable for vehicle applications can still have substantial (up to 75-80%) capacity left. For instance, one million retired 15 kWh/40 kW EDV batteries with an average of 50% remaining power and energy capability can provide 7,500 MWh of energy capacity and 20,000 MW of power capacity, a huge waste if not utilized in secondary applications, such as energy storage for grid support. Sandia National Laboratories released a report on the technical and economic feasibility of such approaches several years ago. Several EDV manufacturers have announced their plans on using old EDV batteries for stationary energy storage.
Current battery management systems (BMS) employ balancing circuits for EDV battery systems. Since EDV usage has certain idle time (parking), battery cell balancing can be performed when the battery systems are not in use. To avoid power loss and reduce costs, usually cell balancing currents are relatively small. Consequently, cell balancing can effectively correct small cell imbalances. However, old battery packs have large variations in their capacities and in other parameters. In stationary usage to support grid operations, battery systems are often used all the time. As a result, new battery pack balancing strategies need to be developed.
Consider a typical scenario of one or more battery systems that support smart grid operations, in which intermittent power sources such as wind farms and photovoltaic (PV) solar electrical systems power a network of battery packs. The battery systems may be connected or connectable to the electric grid, and may also provide power to EDVs, office and residential buildings, factories, and the like. The battery packs may be situated as either stationary and stored in distributed locations, or are EDV batteries which are dynamically configured to be part of the interconnected battery systems.
Retired battery packs, however, are of different types, capacities, ages, and operating conditions, having come from any number of manufacturers, and having different designs. The battery packs are interconnected to form a supporting energy storage system. Charging and discharging such disparate and wide-ranging arrangements of battery packs present unique challenges, as the risk of battery overcharging can result in control schemes that do not take full advantage of the energy storage capability of the entire bank.
Management of used EDV batteries encounters more difficulty than for their primary EDV usage. First, they are typically re-characterized for the remaining capacity, internal impedance, and voltage/state-of-charge (SOC) curves. Secondly, different battery modules, in terms of their sizes, chemistry properties, voltage/current ratings, capacities, etc., are integrated into serial and parallel connections to form a large battery system. Due to their difference in capacity and internal resistance, their charge/discharge rates are controlled properly to avoid some typical detrimental effects, such as overcharge/overdischarge, reduced battery life, etc.
Therefore, there is a need for improved control during charge and discharge of a bank of heterogeneous batteries connected in series and parallel.